In a typical hard disk drive (HDD), servo sectors on the disk are used to provide position information about the location of a magnetic head over a disk surface. A common approach for writing such servo information on one or more disk surfaces in an HDD is referred to as spiral-based self-servo writing, or spiral-based SSW. According to this approach, multiple spiral-shaped servo information patterns (or “servo spirals”) are written on at least one disk surface prior to the SSW process. During the SSW process, a magnetic head of the HDD is positioned relative to a disk surface based on timing and position information in the servo spirals, so that the final servo information (the servo sectors) can be written on the disk surface by the magnetic head.
For an error-free and robust SSW process, the servo spirals used should be precisely written on the disk surface with a predetermined and constant slope. Such servo spirals may be written on the disk surface with an external media writer before assembly of the disk drive, or with a servo writing machine that uses an external precision actuator to position the disk drive actuator with a mechanical push pin through an opening in the disk drive housing. In either case, setup and use of such external equipment for each individual HDD is time-consuming and expensive in the context of high-volume manufacturing.
In light of this, in-drive spiral-writing schemes have been employed, in which an HDD itself writes servo spirals prior to performing the SSW process. For example, a set of coarsely positioned spirals may be written by the HDD while the actuator is moved across a disk surface by applying a suitable open-loop voltage profile, or by using velocity control that is based on back electromotive force (back-EMF) feedback. More precisely positioned sets of spirals can then be written in one or more subsequent closed-loop spiral-writing processes, by demodulating signals from the more coarsely positioned sets of spirals. For example, a set of coarse servos spirals, a set of fine servo spirals, and a set of final servos spirals may be progressively written by the HDD itself in this way. The servo sectors are then written on a disk surface by positioning the magnetic head based on the final, most accurately positioned set of servo spirals.
Generally, the different sets of servo spirals are written on different recording surfaces of an HDD, so that the writing of one set of servo spirals does not overwrite portions of the servo spirals being used to position the magnetic head. However, for an HDD that only has a single data storage surface, a set of servo spirals being written on the single data storage surface necessarily overwrites portions of the preexisting set of servo spirals (i.e., the reference spirals) that are required to control magnetic head position. Crossing a reference servo spiral at a location that has been overwritten, and therefore is unreadable, prevents the servo system of the HDD from collecting position information for the magnetic head, which can adversely affect the ability of the servo system to accurately control magnetic head position. While crossing a single reference spiral at an unreadable location can be a recoverable event for a typical HDD servo system, crossing two or more consecutive reference servo spirals at such unreadable locations is likely to cause an error in the SSW process. Because a blank-disk SSW process performed in a single-surface HDD causes hundreds or thousands of unreadable locations to be formed on the reference servo spirals being used to control magnetic head position, the likelihood of the magnetic head crossing two, three, or more consecutive reference spirals at unreadable locations at least once in the SSW process is extremely high. Therefore, most or all single-surface HDDs employing such a blank-disk SSW process can be expected to fail during the SSW process. As a result, the above-described blank-disk SSW process is not practicable on such single recording-surface HDDs.